Coaxial-line thermistor mount



Oct. 7, 1958 w. J. GALLAGHER 2,

COAXIAL-LINE THERMISTOR MOUNT Filed Sept. 9. 1954 INVENTOR.

United States Patent COAXIAL-LINE THERMISTOR MOUNT William J. Gallagher, Palo Alto, Calif., assignor to Hewlett-Packard Company, Palo Alto, Calif., a corporation of California Application September 9, 1954, Serial No. 454,951

Claims. (Cl. 333-22) accompanying drawings.

Referring to the drawings:

Figure 1' shows a cross sectional view of a coaxial thermistor mount;

Figure 2 shows an equivalent circuit for the apparatus of Figure 1; and

Figure 3 shows a plot. of impedance at the plane of the first design reactive element as a function of frequency on a portion of the Smith chart.

My apparatus consists of a section of coaxial line adapted to be connected to an associated coaxial system. The apparatus incorporates thermistor means which are coupled to the coaxial line by impedance matching means, and which are connected with an external test or measuring circuit.

As illustrated in Figure 1, the section of coaxial line consists of the outer and inner conductors and 11. The inner conductor consists of two aligned portions 12 and 13. Conductor 12 has a portion 14 reduced in diameter at 15 to compensate for the discontinuity introduced by the corresponding change in diameter of the outer conductor'. The extremity 16 of portion 12 is formed to interfit the central conductor of an associated coaxial line conductor (not shown); The inner conductor is held incoaxial alignment by means of a low-loss dielectric spacer 17. The spacer is formed with annular recess 17a and shoulder 17b to compensate for discontinuities in the end of the mount. The spacer is fitted and locked within the bore 19 of the outer conductor and is seated against the adjacent shoulder 18. Aligned holes 21 and 22 are provided in the spacer and inner conductor and serve to accommodate the dielectric lock pin 23. With this arrangement the inner conductor is held' in fixed position by the spacer and with the shoulder formed by the change in diameter .at 15, abutting one side of the spacer.

The inner conductor extremity 16 is surrounded by the contactor sleeve 26. This sleeve consists of a plurality of spring contact fingers 26a which are adapted to make electrical contact with the outer sleeve conductor of the associated coaxial connector. The base 27 of the contactor sleeve can be in the form of a truncated cone. As illustrated, this base seats upon the adjacent end face of the spacer 17 and it is engaged by the turned flange 28 I which is formed by the adjacent portion of the outer conductor.

A sleeve 29 is carried by one end of the outer conductor and is retained by the snap-in spring ring 30. The internal threads 31 on the sleeve 28 are adapted to engage the externally threaded part of the associated connector.

The portion 13 of the inner conductor is formed by the interfitted conducting members 34 and 35. Member 34 is in the form of a sleeve with its one end provided with an outwardly extending flange 34a and its other end provided with an inwardly extending flange 34b. Circumferentially spaced slots 340 permit a certain amount of spring or give whereby the inner periphery of the flange 34b is adapted to press in good electrical contact with the adjacent periphery of the member 35.

The low-loss dielectric wafer 37, formed of suitable material, is interposed between member 34 and the adjacent end face of the inner conductor portion 12. This end face of the inner conductor portion 12 is enlarged by 'an end flange 39 which has a diameter corresponding to the diameter of flange 34a.

The Wafer 37 has thin films of conducting material on its side face, and these films are attached as by solder to the adjacent areas of the flanges 34a and 39.

The conducting member 35 has an end which is seated in member 34. When the conducting member 35 is seated an annular notch 35a is formed on the inner conductor portion 13. As will be presently explained, the member 35 is removable with respect to the wafer and member 34.

That end of the conducting member 35 remote from the wafer 37 is in the form of a reentrant cup which fits over the adjacent end of a cylindrical insulator 36, which, for example, may be in the form of a glass bead. Terminal member 43 is provided with a similarly cupped end portion which fits upon the opposite end of insulator 36.

Surrounding the terminal member 43 there is a sleevelike terminal member 44 which is fitted within an enlarged bore 46 formed in the outer conductor. The. outer terminal member 44 makes electrical contact with the outer conductor on the annular area indicated at 47 and is held in proper position by suitable means such as the spring wire snap-in ring 48.

It is convenient to position a disk-shaped capacitor 49 between. the terminal members 43 and 44 as illustrated. The peripheral terminals 51 and 52 of this capacitor make connection respectively to the terminals 43 and 44.

Nuts 53 and 54 that are threaded upon terminal member 43 facilitate making electrical contact between the capacitor terminal 51 and the annular shoulder 50 of portion 43. They also make contact between the spring 56 and the terminal member 43. Although I prefer to use nuts 53v and 54 to make the connections indicated because of ease in assembly, it is readily seen that this could be done in other ways, for example, by soldering.

Two thermistors are mounted in the annular space surrounding theinsulator 36. Thus one thermistor 57 has its leads 58- and. 59 connected respectively to the conductor member 35 and inner periphery of the terminal member 44. The second thermistor 61 has its leads 62 and 63 connected respectively to the conductor 35 and the terminal member 43.

For convenience in making connections to an external test or measuring circuit, a cap 67 is threaded upon the outer conductor and serves to mount a connector 68. This connector can be of conventional construction, including the outer sleeve conductor which connects directly with the outer conductor 10 through the cap 67,

' and a center conductor which has an inner extremity 69.

Means are then provided to provide an electrical connection between the said center conductor and the terminal member 43. Spring 56 provides the electrical connection in the embodiment shown.

The member 35 is removable with respect to the wafer and member 34 to permit mounting of the thermistors before the members are engaged. This also provides a means for replacing thermistor elements.

In Figure 2 I have shown a lumped equivalent circuit for the apparatus of Figure 1. Lines 71 and 72 represent the outer conductor and inner conductors of the coaxial section, respectively. Capacitor 73 represents the capacitance interposed in series with inner conductor by wafer 37. Capacitor 74 represents the shunt capacity introduced by end flange 39 and the flange 34a. The inductance 76 represents the series inductance introduced by the notch 35a. Capacitor 77 represents the distributed shunt capacitance between the portion 35 and the adjacent outer conductor 10. Inductors 75, 79, 81 and 82 represent the inductance of the thermistor leads. Resistors 83 and 84 represent the thermistor bead resistance. Capacitors 86 and 87 represent the capacity between the thermistor leads, and the capacitor 88 represents the capacitance across insulator 36. The capacitor 89 represents the disk-shaped capacitor 49 which is placed between the terminal members 43 and 44. The direct or audio frequency current connections are shown as 91 and 92. The portion of the circuit represented by block 93 represents a network analysis of the thermistor mount and the circuit represented by block 94 represent the matching network which provides operation over a broad frequency band.

The wafer 37 is usually chosen so that it isolates portion 12 from the portion 13 of the inner conductor as far as direct and audio frequency currents are concerned. This wafer offers negligible impedance to a microwave energy flowing along the coaxial system. Similarly the disk capacitor 49, represented by capacitor 89 in the equivalent circuit, is chosen so that it offers a short circuit to the high frequency current and an open circuit to the direct and/ or audio frequency current.

Referring to Figure 2, capacitors 73 and 89 are short circuits at microwave frequencies. As a consequence, the thermistors present a parallel impedance to the coaxial system. If the thermistors have a resistance of 100 ohms each, a 50 ohm line is matched by the two thermistors. Capacitors 73 and 89 are open circuits to direct and audio frequency current. Referring to Figure 2, it is seen that the thermistors are in series to direct and audio frequency current. Assuming the above values, they would present an impedance of 200 ohms.

As is well known, the impedance of an element in a coaxial system is dependent upon the plane of reference. Therefore the location along the axis of the flanges 34a and 39 andnotch 35a, which form the matching network is important. They must be located such that when the impedance of the matching network is combined with the impedance of the thermistors and mount, a match between the mount and coaxial system will result.

Referring more particularly to Figure 1, the direct current path is through the center conductor of the connector 68, along the spring 56, through the inner terminal member 43, through thermistor 61, through thermistor 57, to the outer terminal member 44 and to the outer conductor of the connector 68. The microwave current flows along the inner conductor portions 12, 13 and 35 through the thermistors 57 and 61, and back to the outer conductor 10, in one case directly through the terminal member 44, and in the other case through terminal member 43, capacitor 49 and terminal member 44 to the outer conductor 10. With the mount properly matched all the microwave energy is converted to heat in the thermistors. The microwave energy is measured by the method of substitution of energy which is well known. Briefly, it consists of supplying direct or audio frequency power to the thermistors from a balanced bridge circuit to maintain constant resistance, in this case 100 ohms each. As high frequency energy is supplied the direct or audio frequency power requirements are reduced. The reduction is a measure of the high frequency energy. The advantage of this method is apparent. The resistive compo constant standing wave ratio of 1.5.

nent of the thermistors is constant and mount matching is simplified.

Apparatus was constructed as shown in Figure 1 and dimensioned as follows: Outer conductor had an inside diameter of 0.391 inch and an outside diameter of 0.781 inch, the bore adapted to receive the dielectric spacer 17 was 0.230 inch long and 0.455 inch in diam eter, the portion adjacent the thermistor was 0.080 inch long and 0.203 inch in diameter, the main bore was 0.600 inch long. Portion 12 was 0.950 inch long, the flange was 0.205 inch in diameter and 0.010 inch thick, the portion adjacent the flange was 0.170 inch in diameter and 0.270 inch long, the portion which interfits the bead was 0.1205 inch in diameter and 0.475 inch long and the portion which interfits the inner conductor of the associated coaxial system was 0.065 inch in diameter; member 34 was 0.120 inch long and had a flange 0.205 inch in diameter and 0.010 inch thick, the inside diameter of the inwardly extending flange was 0.090 inch, the member was slotted with four slots 0.010 inch wide and 0.100 inch deep; member 35 was 0.235 inchlong, the cup was 0.040 inch deep and 0.125 inch in diameter; the glass bead was 0.090 inch long and 0.125 inch in diameter; the terminal member had a cup which was 0.020 inch deep and 0.160 inch in diameter; the wafer had a capacity of 2000 fg the disk-shaped capacitor 49 had a capacity of 550 ,unfi; the thermistors were known by manufacturers specifications as VECO 32A 5.

Figure 3 is a plot of admittance as a function of frequency for the frequency range 10 mc. to 10 kmc. on a portion of a Smith chart. The circle represents a It is apparent that this thermistor mount has an acceptable voltage standing wave ratio over a broad band of frequencies.

Iclaim:

1. In a coaxial line thermistor mount, outer and inner concentric conductors forming a section of coaxial line and adapted to be connected to an associated coaxial system, the inner conductor comprising first and second aligned portions, dielectric means providing a cont1nuous path for high frequency energy through sa1d first and second aligned portions, two thermistors, a pair of terminals adapted to be connected to an external measuring circuit, means forming a high frequency by-pass capacitor connected between the terminals, the leads of the first thermistor being connected to the second portion of the inner conductor and one of said terminals, the leads of the second thermistor being connected to the second portion of the inner conductor and the other one of sald terminals, and impedance matching means formed on said first and second portions serving to match the impedance of said thermistors and mount to that of the associated coaxial system.

2. In a coaxial line thermistor mount, outer and inner concentric conductors forming a section of coaxial line and adapted to be connected to an associated coaxial system, the inner conductor comprising first and second aligned portions, dielectric means providing a continuous path for high frequency energy through said first and second aligned portions, two thermistors, a pair of terminals adapted to be connected to an external circuit, means forming a high frequency by-pass capacitor connected between the terminals, the leads of the first thermistor being connected tothe second portion of the inner conductor and one of said-terminals, the leads of the second thermistor being connected to the second portion of the inner conductor and the other one of said terminals, means formed on said first and second inner conductor portions introducing a capacitive element between the inner and outer conductor, and means formed on said second portion introducing an inductive element in series with the inner conductor, said capacitive and inductive elements forming an impedance matching network whereby an impedance match is obtained between the thermistor mount and the associated coaxial system.

3. Apparatus as in claim 2 wherein said second portion of the inner conductor comprises inner and outer interfitting members, the outer members being slotted and having one end thereof adapted for electrical contact with the peripheral portion of the inner member.

4. Apparatus as in claim 2 wherein said means forming a capacitive element between the inner and outer conductor comprises flanges formed on said inner conductor, wherein the means forming an inductive element comprises a notch interrupting the periphery of said second portion of the inner conductor.

5. In a coaxial line thermistor mount, outer and inner conductors forming a section of coaxial line and having one end thereof adapted to be connected to an associated coaxial system to receive high frequency energy, the inner conductor consisting of first and second aligned portions, the first portion being adapted for connection with the associated coaxial system and the second portion being adapted for connection with the thermistor means, the second portion comprising inner and outer interfitting members, the outer member being slotted and having one end thereof adapted for electrical contact with the peripheral portion of the inner member, a wafer of dielectric material interposed between the opposing end faces of said portions, said wafer serving to insulate said portions one from the other to direct an audio frequency current and providing a low impedance path one to the other to the high frequency current, a pair of terminals adapted to be connected to an external circuit,

a high frequency by-pass capacitor connected between said terminals, one of said terminals being coaxial with said inner conductor and spaced axially with respect to the inner member of the second portion, an insulator interposed between said inner member and said one terminal, a first thermistor disposed adjacent said insulator and having its one terminal lead connected to the said inner member and its other lead connected to one of said terminals, a second thermistor spaced circumferentially from said first thermistor and likewise disposed adjacent said insulator, one terminal lead of the second thermistor lead being connected to said inner member and the other being connected to the other one of said terminals whereby the two thermistors are connected in series with the terminals, said section of coaxial line serving to supply high frequency energy to said two thermistors in shunt, the adjacent parts of the first portion and the outer member of the second portion formed with an enlarged diameter to thereby form a capacitive element between the same and the adjacent surrounding portion of the outer conductor, and the inner member of the second portion having a shoulder formed thereon which is spaced axially from the adjacent end portion of the outer member to thereby form an annular notch providing an inductive element in series with the inner conductor, said capacitive and inductive elements providing an impedance matching network which matches the impedance of the mount to that of the associated coaxial system.

References Cited in the file of this patent UNITED STATES PATENTS 2,777,995 Henning Jan. 15, 1957 

